This invention relates to a gate turnoff thyristor which is rendered non-conductive in response to a negative gate signal applied thereto.
A rapid and remarkable technical developement has progressed in semiconductor high-power switching devices, particularly in power thyristors. As is known, the power thyristor cannot be turned off by a gate signal and the turn-off of the power thyristor needs an additional forced commutation circuit, making it difficult to apply the power thyristor to a chopper and the like.
Another high-power switching element, which has been known, is a power transistor which can be turned off by a negative base bias applied thereto. The power transistor is defective in that a continuous flow of the forward base current is needed in order to keep the power thyristor conductive, and that the current capacity of the power transistor is less than that of the power thyristor.
Also, a gate turn-off (GTO) thyristor has been known as a high power switching element, adaptable for switching devices such as inverters, choppers and the like, without the forced commutation circuit. Because of no need of the bulky forced commutation circuit, the GTO thyristor has attracted an attention in the field of high power electronics. However, the GTO thyristor has a drawback; it has an extremely small anode current capacity. This arises from generation of heat due to anode current being locally concentrated when the thyristor is rendered nonconductive. To describe in greater detail, when the thyristor is turned off, anode current flow concentrates through a local portion of the thyristor with the resultant prominent increase in current density at the local portion. Generation of heat there sometimes leads to the damage of the thyristor. For this reason, anode current which can be allowed to pass through the thyristor is subjected to a certain limitation, for example, about 50 to 200 amperes. This small value is about one-tenth of that which is possible with a thyristor in general use.
Among the factors to determine the anode current capacity, i.e. the maximum anode current, the most important factor is a sheet resistance .rho..sub.sb (.OMEGA./ ) of a base layer adjacent to a cathode layer. The reason for this follows. Reference is made to FIG. 1 illustrating in structural and schematic form, a GTO thyristor with a turn-off circuit. As shown, the GTO thyristor has a semiconductor body having four layers, a P anode layer 12, an N base layer 14, a P base layer 16 and a cathode layer 18. These four layers are provided with cathode, gate and electrodes 20, 22 and 24, respectively. The cathode and the anode electrodes 20 and 24 have a series circuit connected therebetween having a power source 26 and a resistor 28. The cathode and the gate electrodes 20 and 22 have a bias power source 30 connected therebetween via a switch 32. In turning off the thyristor thus constructed and connected, a switch 32 is turned on at time t.sub.1 in FIG. 2 thereby to bias the gate of the thyristor to a negative potential. That is, a negative pulse (turn-off pulse) Vg as shown in FIG. 2A is applied to the gate electrode 22. Upon receipt of the negative pulse Vg, a gate current Ig as shown in FIG. 2B flows through the P base layer 16. The gate current Ig is obtained by shunting the anode current I.sub.A to some extent but is relatively large in actuality. For example, when the anode current I.sub.A is approximately 400 (A), the gate current Ig ranges from 100 to 200 (A). As the gate current Ig starts to flow, the anode current I.sub.A substantially linearly decreases after lapse of a predetermined time (storage time t.sub.s) and becomes zero at time t2, as shown in FIG. 2C. Thereafter, even if the turn-off pulse Vg becomes zero at time t3, the anode current I.sub.A is kept zero. In this manner, the GTO thyristor is turned off. The maximum allowable gate current Ig of the P base layer 16 substantially determines the maximum value of the anode current I.sub.A. Accordingly, it is eagerly desired to improve the maximum allowable value of the gate current Ig in order to increase the allowable anode current I.sub.A. The maximum value of the gate current Ig to flow into the P base layer 16 is substantially determined by the sheet resistance .rho..sub.sb (.OMEGA./ ) of the P base layer 16.
Thus, the sheet resistance .rho..sub.sb (.OMEGA./ ) of the P base layer 16 is an important factor to determine the maximum allowable gate current Ig, that is to say, the maximum anode current I.sub.A.
In contrast with the general type GTO thyristor as shown in FIG. 1, a special type GTO thyristor is known called a gate-assisted turn-off thyristor (GATT). However, the sheet resistance .rho..sub.sb (.OMEGA./ ) of the anode current capacity of the GATT is less important than that of the general type GTO. In more particular, the GATT is comprised of a P anode layer 42, an N anode layer 44, a P base layer 46 and an N cathode layer 48, as shown in FIG. 3. The layers 48, 46 and 42 are connected to cathode, gate and anode electrodes 50, 52 and 54, respectively. A power source 58 connected in series with a resistor 56 is connected between the cathode and anode electrodes 50 and 54 and a forced commutation circuit 60 is also connected between these electrodes 50 and 54. A couple of series circuits, which include a switch 62 and a positive bias power source 66, and a switch 64 and a negative bias power source 68, is inserted between the gate and cathode electrodes 52 and 50.
In turning off the GATT thus constructed, a gate pulse current Ig1 is fed to the thyristor (not shown) of the forced commutation circuit at time t1, as shown in FIG. 4A. Upon receipt of the gate pulse, the thyristor is turned on to cause a current I.sub.A flowing the anode to cathode path of the thyristor through the resistor 56 to shunt as a current Ic (FIG. 4B) into the commutation circuit 60. Accordingly, the anode current I.sub.A substantially linearly decreases with time. On the other hand, the shunt current Ic increases with time. When the anode current I.sub.A becomes substantially zero, i.e. at time t2, the switch 64 is turned on. Accordingly, the gate electrode 52 is biased negatively, so that the anode current I.sub.A becomes approximately zero flow through the P base layer 46 to the gate electrode 52. The gate current at this time is denoted as Ig2 in FIG. 4D. At time t3, when the anode current I.sub.A approximates zero, the gate current Ig1 flowing into the commutation circuit 60 is shut off. The shunt current Ic is maximized shortly after the anode current I.sub.A is zeroed and then linearly decreases with time. If the pulse Ig2 disappears at time t4, no anode current I.sub.A flows. In this way, the GATT is turned off.
The current Ig2 flows into the P base layer 46 when the anode current I.sub.A becomes substantially zero. Therefore, it is very small and is much smaller than the current flowing into the P base layer 16 (FIG. 5) in the case of the GTO thyristor.
Therefore, in the case of the GATT, the sheet resistance .rho..sub.sb (.OMEGA./ ) of the P base layer 46 is not an important factor to determine the anode current I.sub.A. To repeat, the sheet resistance of the P base layer is a very important factor to determine the anode current in the case of the GTO thyristor but is not in the case of the GATT. That is, the sheet resistance .rho..sub.sb (.OMEGA./ ) of the P base layer 46 is a very important factor when the anode current especially in the GTO is taken into consideration.